The present disclosure relates to a variable-resistance memory device including a storage element with a resistance varying due to application of a signal set at one of different polarities to opposite ends of the storage element and relates to a method for driving the variable-resistance memory device.
There is known a variable-resistance memory device having a storage element included in every memory cell of the variable-resistance memory device to serve as a storage element with a resistance varying due to injection of conductive ions into an insulation film or withdrawal of such ions from the insulation film. For more information on the variable-resistance memory device, refer to documents such as K. Aratani, etc. “A Novel Resistance Memory with High Scalability and Nanosecond Switching,” Technical Digest IEDM 2007, pp. 783 to 786.
The storage element has a laminated structure in which a layer for supplying the conductive ions and the insulation film are formed at locations between two electrodes.
Each memory cell cited above is configured to include the storage element and an access transistor which are connected to each other in series at locations between first and second lines drivable by adoption of an active matrix driving method. Since the memory cell has one access transistor (T) and one storage element with a variable resistor (R) as described above, the memory cell is referred to as a 1T1R memory cell.
In addition, the variable-resistance memory device including such 1T1R memory cells is referred to as a ReRAM (Resistance Random Access Memory).
As described in documents such as K. Aratani, etc. “A Novel Resistance Memory with High Scalability and Nanosecond Switching,” Technical Digest IEDM 2007, pp. 783 to 786, in the ReRAM, the magnitude of the resistance is associated with a state in which data has been written into the storage element and a state in which data has been erased from the storage element. Data is written into the storage element and erased from the storage element by applying a pulse having a short duration in the ns (nano-second) order. Thus, in the same way as the RAM (Random Access Memory), the ReRAM is capable of carrying out operations at a high speed and serves as an NVM (Non-Volatile Memory) which draws much attention.
In the variable-resistance memory device such as the ReRAM and an MRAM, a high-resistance state and a low-resistance state are recognized as a state in which data has been written into the storage element and a state in which data has been erased from the storage element. It is thus necessary to generate a reference current as described in documents such as Japanese Patent Laid-open No. 2010-049730. The MRAM is a spin injection magnetic resistance RAM.
In order to effectively suppress process variations and effects such as temperature-characteristic following effects, the reference current is generated by a reference cell which makes use of the same elements as the memory cell utilizing the reference current as a current for recognizing stored information.
In addition, in order to obtain the reference current, the reference current is generated by making use of two or more single units or a combination of the single units. The two or more single units are single units in the high-resistance and low-resistance states of the memory cell. For the two different types of the states, data is read out in the same read current direction.